In the art of making color hard copies, thermal transfer processes, electrophotography, and ink jet processes are being studied energetically at present. Among these, heat transfer processes have many advantages because heat transfer apparatus are easily maintained and operated, and the apparatus and expendable supplies are relatively inexpensive. Processes and apparatuses for heat transfer process are disclosed, e.g., in U.S. Pat. No. 4,621,271.
There are two common methods of the thermal transfer process. One comprises heating a thermal transfer dye donating material having a heat fusible ink layer provided on a base film with a thermal head to melt the ink, and recording the image with the molten ink on a thermal transfer image receiving material. The second comprises heating a thermal transfer dye donating material having on a base film a color material layer containing a thermal transfer dye with a thermal head to transfer the dye into a thermal transfer image receiving material.
In the second process involving the thermal transfer, the amount of dye transferred can be varied by changing the energy applied to the thermal head. This makes gradation of the color transferred possible, which is especially advantageous in the full color recording of high quality images. The thermal transfer dyes used in this process, however, have various restrictions, and quite few dyes can satisfy all the required properties.
The required properties are, for example, spectral characteristics suitable for color reproduction, facility for thermal transfer, fastness to light and heat, resistance to various chemical reagents, no or slight decrease in sharpness, negligible retransfer of images, and facility for preparing a thermal transfer dye donating material.
Of particular interest were, three-color combinations of dyes, namely yellow, magenta and cyan dyes, which enabled the formation of full color images giving excellent color reproduction and light resistance. In addition, heat transfer dye donating materials that do not crease if deformed and do not adhere by the fusion to the heat transfer image receiving material when heated with the thermal head are also desired.
In the full color images obtained using known three-color combinations of dyes, namely yellow, magenta and cyan dyes, as thermal transfer dyes, neither the color reproduced nor the light fastness were satisfactory. In addition, deformation of these materials generates creases and the application of heat resulted in fused adhesion of the dye donating material to the heat transfer image receiving material.